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I'm replacing an old brick hearth with tile, and wondering the best way to approach this. I've removed most of the brick except for a 48x60" area, and will be using 12x24" tiles. My plan is to put the tiles on top as it brings the height up to the level I want. The sides of this will also be made of the same tiles.

brick pad

I took a picture of the tiles laid over while I was trying to figure out sizing:

view of hearth with tile laid on top

The brick overall is level, but uneven from the grout joints. I've knocked down any high points (especially on the outside back edges, where there was a floor-to-ceiling brick wall on top). I suspect it's been installed for somewhere between 25 and 35 years, and showed no signs of cracks or anything -- everything you see here is from me deconstructing it.

I will be using a leveling system with wedges to ensure the tiles are true. This will have a propane stove on top when complete:

brick hearth with stove

  • To fix the loose brick around the edges can I use some fast-setting concrete mix, the same mortar for the tiles, or do I need to build a form and completely fill it in to be square?

  • For the tiles, do I need to put a flat coat over top first, can I lay tiles directly on the brick with "large tile mortar" (medium bed mortar), or should I use an uncoupling membrane?

  • Anything else I should do?

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  • "To fix the brick around the edges" The only bad spot I see is where the straight edge transitions into the curve. If your project allows, why don't you top coat everything and apply tile to the entire construction, base and top? Commented Nov 1, 2023 at 17:13
  • @DelphicOracle I just meant the loose bricks, and yeah, I'll do a better job filing in that curved bit. I can definitely do a top coat, but it adds a lot of work -- I guess I'm wondering if it is beneficial or necessary, since most medium bed mortars seem to do 3/4" without sagging?
    – gregmac
    Commented Nov 1, 2023 at 18:28
  • Do the bricks readily absorb water? Or is there some kind of seal on them?
    – popham
    Commented Nov 1, 2023 at 21:46
  • @popham I don't think they do. They're exterior bricks, same as what is on the outside of the house. No seal, they're directly mortared on the slab. They've been there for a couple decades, and survived one flood I know about.
    – gregmac
    Commented Nov 1, 2023 at 22:10
  • The back bricks look like they've come loose. What happened there? Are you sure that the substrate is reliable?
    – popham
    Commented Nov 1, 2023 at 23:25

2 Answers 2

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Tile Over Brick:

The brick needs to be unsealed for the thinset to make a good bond. Check to be sure that the brick readily absorbs water. And get the brick damp (not puddles of water) before tiling so that it doesn't suck the water out of your thinset so eagerly.

You want an LFT thinset for that size of tile. With the extra depth at the brick grout lines, I would recommend the LFT thinset even if you were using smaller tiles. The LFT thinset is a "medium bed mortar" designed for thicker applications than ordinary thinset. This should compensate for the dips at your brick grout lines. I'm 90% sure that there's no such thing as an unmodified LFT thinset, but be sure it's modified thinset. It will have "ANSI 118.4" conformance written somewhere on the bag, but the word "modified" in marketing materials is sufficient. The cheapest LFT thinset is almost certainly ANSI 118.4 conformant.

I would trowel LFT thinset into the brick grout lines a couple hours before tiling. This would allow the thinset in the grout lines to undergo its shrinkage before getting topped with fresh thinset as you lay your tile. That's me being a little obsessive probably, though. You should be fine applying to your brick without any preliminary filling as long as your thinset is deep enough.

Those large format tile take lots of thinset. You should probably be using a 1/2" square notch trowel to get sufficient thickness for your 12x24s.

Tile Substrate:

Installing brick to get your square pedestal seems like a lot of unnecessary labor. The big drawback of forming a concrete edge is getting a good bond with the existing brick and concrete floor. Rather than drilling and epoxying rebar to secure the concrete, I suggest using the same technique used for putting a preslope on top of concrete: Install forms, mix your concrete, trowel a layer of modified thinset on the face of the adjacent concrete and brick, and immediately place your concrete before the thinset cures or dries. The existing concrete slab and brick can't be sealed, of course.

Notes on specs:

  • ANSI 108.02 and the TCNA Handbook require 80% coverage for tile installed in dry areas. Without any bond at the brick's grout lines, you can still meet this spec easily.

  • LFT mortar requires a 1/2" thickness or less. Assuming that you use a 1/2" square notch trowel, the square notch displaces 50% of the mortar with its low spots. When you collapse the ridges of this mortar during installation, then, the 1/2" ridges should collapse to a uniform thickness of 1/4". The LFT mortar's maximum thickness of 1/2", then, allows for brick grout lines shallower than 1/4" while still meeting the maximum thickness constraint.

  • I can't rationalize the thinset bonding layer between fresh concrete and old concrete based on any authoritative sources. If the current brick survived in its current state while bonded by mortar to the concrete and each other, then it stands to reason that the fresh concrete bonded by modified thinset will enjoy similar longevity.

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According to this site

https://www.mapei.com/us/en-us/training-and-technical-service/tech-talk-blog/detail/mapei-blog/2020/08/03/can-mortars-be-used-to-patch-or-level-floors

you should top coat the brick to provide a smooth surface to support your tile. Topping mortar with mortar will create bonding problems between the mortar layers.

Sagging refers to the viscosity of the mortar to support itself and maintain an edge when troweling to allow the tile to bed properly. This assumes that you have a flat surface that you are troweling into.

You are proposing to use the mortar as both a filler and a bonding agent for the tile. The mortar will cure at different rates relative to the existing mortar joints and brick. This will lead to cracking. To avoid transmitting this to the tile, you should have a decoupling layer. You see where this is going?

Do the top coat.

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  • Honestly this is a bit overwhelming. There's an absurd number of products to use here -- patches, self-levelers; cement- vs calcium-aluminate-based -- and I am just not sure what to choose. Is the top coat a decoupling layer, or are you saying I also need to use something like Schluter Ditra on top of that? I am not proposing any solution here, I am just trying to figure out something I can get done in the next few days. Another option may be ripping the rest of this brick out and building a frame from PT wood and putting cement board on top?
    – gregmac
    Commented Nov 1, 2023 at 21:33
  • Tile in dry areas only needs 80% coverage by ANSI 108 and by the TCNA Handbook. Fussing with the brick grout lines seems silly. Settling for no thinset there, he's still easily conformant. And if the grout lines are shallower than 1/4", then a 1/2" square notch trowel keeps him within the LFT mortar's 1/2" thickness upper limit. There's no evidence of movement of the brick whatsoever. An uncoupling membrane seems unnecessary.
    – popham
    Commented Nov 1, 2023 at 22:43

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